The present invention relates generally to lasers, and specifically to stabilization of lasers operating in a single mode.
FIG. 1 is a schematic diagram showing operation of a lasing system 18, as is known in the art. System 18 comprises two mirrors 20 and 22 separated by a distance L. In order for system 18 to laser i.e., to resonate, at a wavelength xcex, a medium 24 between mirrors 20 and 22 must provide gain, and an effective optical path length Leff between the mirrors must be an integral number of half-wavelengths. Quantitatively,
Leff=nLxe2x80x83xe2x80x83(1a)
so that
mxc2x7xcex/2=nLxe2x80x83xe2x80x83(1b)
or
f=mxc2x7c/(2nL)xe2x80x83xe2x80x83(1c)
wherein m is a positive integer, n is a refractive index of medium 24, f is the frequency corresponding to the wavelength xcex, and c is the speed of light.
From equation (1c), a separation xcex94f of lasing frequencies is given by
xcex94f=c/(2nL)xe2x80x83xe2x80x83(2)
Each such lasing frequency corresponds to a longitudinal cavity mode. Since f=c/xcex, xcex94f≅xcexxe2x88x92cxc2x7xcex94xcex/xcex2 so that equation (2) can be rewritten to give a separation xcex94xcex of lasing wavelengths:
xcex94xcex≅xcex2/(2nL)xe2x80x83xe2x80x83(3)
FIG. 2 is a graph of intensity I vs. wavelength xcex illustrating cavity modes for system 18, as is known in the art. A curve 30 represents an overall gain of medium 74 in system 18. Peaks 32A and 32B, with separation xcex94xcex, show the cavity modes present in system 18, each node corresponding to a different value of m. As is evident from FIG. 2, there are many possible cavity modes for system 1X.
Optical communications within fiber optic links require that the laser carrier have as small a frequency spread as possible, particularly when multiple wavelengths are to be multiplexed on a single fiber. Thus, for efficient communication only one cavity mode should be used, and optimally the frequency spread within the mode should be minimized. Typically, methods for stabilizing the frequency of the laser include utilizing distributed feedback (DFB) lasers and/or distributed Bragg reflectors (DBR). DFB lasers have a frequency-selection grating built into the laser chip, the grating being physically congruent with the gain medium. The grating in a DBR laser is external to the gain medium. The gratings in DFB and DBR lasers are part of the semiconductor material, which is unstable, DFB and DBR lasers were therefore typically externally stabilized utilizing an external wavelength reference in order to achieve good stability.
FIG. 3 shows the effect of adding a tuning element such as a fiber grating to system 18, as is known in the art. A curve 34 shows the resonance curve of the fiber grating, which has a bandwidth xcex94xcexG of the same order as xcex94xcex, the separation between the longitudinal cavity modes. If the grating is optically coupled to system 18, then mode 32A is present, and other modes such as mode 32B, are suppressed.
FIG. 4 is a schematic diagram showing a gain medium 38 coupled to a fiber grating 50, as is known in the art. Gain medium 38 is formed from a semiconducting gain element 44 having a laser gain region 42. Light from region 42 exits from a facet 56 of region 42 to a medium 46, and traverses medium 46 so that a lens 48 collects the light into a fiber optic 52. Fiber grating 50 is mounted in fiber optic 52, which grating reflects light corresponding to curve 34 of FIG. 3 back to region 42. The mirrors of the laser cavity comprise a rear mirror which in this example is a back facet 57 of the semiconductor gain element, and an output coupling mirror which in this example is the fiber grating. The rear and output coupling mirrors could also be reversed. In the reversed configuration the rear mirror would be the fiber grating and the output coupling mirror would be back facet 57 of the semiconductor gain element. In the reversed configuration the detector would preferably be positioned behind the fiber grating. It is desirable to eliminate parasitic reflections due to surfaces and interfaces internal to the cavity. To eliminate parasitic reflection from the facet of the interfaces internal to the cavity. To eliminate parasitic reflection from the facet of the semiconductor closest to the fiber grating, in this case facet 56, that facet is usually anti reflection coated. It is also useful to anti reflection coat a tip 49 of the fiber closest to the semiconductor gain element to again reduce parasitic reflections. Preferably, grating 50 is written directly at the end of the fiber optic facing the laser. Alternatively, a length Lf of a fiber 63 is interposed between lens 48 and fiber optic grating 50. Thus region 42, medium 46, fiber optic 63 and grating 50 form a resonant system 60 corresponding to region 24 of FIG. 1. This architecture is generally known in the art as an external cavity laser or more specifically as a fiber grating laser (FGL). System 60 has an effective optical path length Leff given by:
Leff=n1xe2x88x92L1+n0xc2x7L0+nfxc2x7Lf+ngxc2x7Lgefxe2x80x83xe2x80x83(4)
wherein n1 is a refractive index of region 42;
L1 is a length of region 42;
n0 is a refractive index of medium 46;
L0 is a length of medium 46;
nf is a refractive index of fiber 63;
Lf is the length of fiber 63.
ng is a refractive index of grating 50; and
Lgef is an effective length of grating 50.
Replacing the optical path length nL of equation (1b) by that given by equation (4) leads to the following equation giving cavity modes for the system of FIG. 3:
m xcex/2=(n1xc2x7L1+n0xc2x7L0+nfLf+ngxc2x7Lgef)xe2x80x83xe2x80x83(5)
In constructing system 60, it is necessary to adjust and maintain the positions of curve 32A and 34 to have their peaks at the same wavelength. Changes in temperature and/or changes in injection current into region 42 and/or mechanical changes affect one or more parameters of the optical path length given by equation (4). Such changes can thus cause mode hopping, which refers to the phenomena whereby mode 32 A shifts underneath resonance curve 34 of the fiber grating. When that shift is large enough, an adjacent mode will at some point experience a larger gain and start to lase. These mode hops occur underneath the resonance curve of the fiber grating (curve 34 in FIG. 3) resulting in wavelength shifts and intensity noise when the mode hops. For example, referring to FIG. 3, mode 32A.
U.S. Pat. No. 4,786,132 to Gordon, whose disclosure is incorporated herein by reference, describes a semiconductor laser diode coupled to a single mode optical fiber. The fiber comprises a built-in Bragg reflector grating which reflects of the order of 50% of the light from the laser back to the laser. The reflected light provides feedback to the laser so that the laser produces a single frequency output.
U.S. Pat. No. 5,077,816 to Glomb et al., whose disclosure is incorporated herein by reference, describes a narrowband laser source, a portion of the light from which is supplied to a resonant grating region in a fiberoptic, external to the laser. The current through the laser is dithered, causing the frequency of the laser to dither. The correspond in dithered light intensity transmitted by the grating is used in order to adjust the current through the laser so as to maintain the frequency of the laser at the resonant frequency of the grating.
U.S. Pat. No. 5,706,301 to Lagerstrom, whose disclosure is incorporated herein by reference, shows a laser control system which uses a fiber optic grating as a resonant control element. A difference in light intensity between laser light passing through the grating, and light which does not pass through the grating is measured, and the difference is used in order to vary the temperature of a laser generating the light, so as to maintain the frequency of the laser at the resonant frequency of the grating.
It is an object of some aspects of the present invention to provide improved methods and apparatus for stabilization of the oscillating frequency of a laser.
In preferred embodiments of the present invention, a laser assembly comprises a semiconducting laser, a fiber grating, and an optical path coupling the laser and the grating. In order to stabilize the output of the laser assembly in a single cavity mode, the effective length of an optical cavity of the laser assembly is modulated about a man value by varying the optical length of at least one of the elements forming the laser assembly. A corresponding modulation of an intensity of the laser output is measured and is coupled in a feedback loop to control the optical length of the element in the laser assembly so as to provide the desired mode stabilization.
Most preferably, the laser and the grating are assembled on an optical bench. The fiber grating is tuned so that only a single resonating mode of the laser assembly is capable of sustaining oscillation, and an output of the single mode is provided via the fiber grating. The laser assembly acts as a resonant cavity, and the fiber grating acts as a wavelength reference within the resonant cavity. The effective cavity optical length is a function of an optical length of the semiconducting laser, an effective optical length of the fiber grating, and an optical length of the optical path coupling the grating and the laser. One or more of these lengths are controlled in order to stabilize the output of the laser assembly.
A difference between the modulation of the effective cavity optical length and the modulation in intensity, preferably a difference in phase, is used as an indicator of where the cavity mode of the laser assembly is oscillating relative to the resonant curve of the fiber grating. The indicator is used within the feedback loop to maintain the oscillation at the peak of the resonant curve of the fiber grating, by varying the mean value of the effective cavity optical length of the laser assembly. Choosing at least one optical length forming the effective cavity optical length and varying the chosen optical length in order to stabilize the laser output is an adaptable and accurate way to stabilize the laser.
In some preferred embodiments of the present invention, the effective optical length of the laser assembly is modulated by periodically varying a temperature of one of the elements of the assembly about a mean temperature, thereby causing the assembly to expand and contract. The mean value of the effective optical length is varied by varying the mean temperature of the element.
In some preferred embodiments of the present invention, the semiconducting laser is mounted on a thermally insulating element, and an electric heating element is placed between the laser and the insulating element. The insulating element has the effect of ensuring that a maximal temperature increase in the laser is attained for a given input electrical power to the heating element. Thus the heating element may be used to modulate the temperature and to change the mean temperature of the laser (or of one or more other elements within the laser assembly) in a controlled manner, and thus to modulate and change the mean value of the optical length of the one or more elements.
In some preferred embodiments of the present invention, at least some of the elements comprising the laser assembly are coupled to a thermoelectric cooler, which enables the temperature of the coupled elements to be changed. Changing the temperature of the fiber grating enables its resonant wavelength to be adjusted in a controlled manner.
There is therefore provided, according to a preferred embodiment of the present invention, apparatus for stabilizing an output wavelength of a laser assembly, including:
a plurality of optical elements coupled together so as to form a laser cavity resonating in a single mode dependent upon an optical length of the cavity;
an optical length changer which varies an optical length of at least one of the optical elements so as to vary accordingly the optical length of the cavity;
a detector which monitors the output of the laser assembly responsive to the variation in the optical length of the at least one of the optical elements; and
a stabilizer which responsive to the measured output from the detector supplies a control signal to the optical length changer to control an optical length of at least one of the optical elements, so that the cavity resonates stably at the output wavelength in the single mode.
Preferably, the optical length changer includes a heating element which varies a temperature of at least one of the optical elements, thereby varying the optical length of the at least one of the optical elements.
Preferably, the heating element includes an electric heating element, which is supplied by a direct current component and an alternating current component in order to alter and modulate a mean temperature of at least one of the optical components.
Further preferably, the heating element dissipates a modulated power having a peak value less than or equal to about 200 mW.
Preferably, the heating element includes a heat insulating element, which selectively directs heat to the at least one of the optical elements.
Preferably, the heat insulating element includes silicon dioxide.
Preferably, the plurality of optical elements includes a semiconductor gain region and a fiber grating having a resonant wavelength.
Preferably, the at least one of the optical elements whose length is varied by the optical length changer includes the semiconductor gain region.
Preferably, the plurality of optical elements includes a medium optically coupling the semiconductor gain region and the fiber grating, and the at least one of the optical elements whose length is varied by the optical length changer includes the medium.
Preferably, the optical length of the cavity is varied to substantially lock the single mode of the cavity to the resonant wavelength.
Preferably, the optical length changer varies the optical length of the at least one of the optical elements so as to correspond to the resonant wavelength.
Further preferably, the apparatus includes a thermal transfer element which varies a temperature of at least one of the optical elements, thereby varying the optical length of the cavity.
Preferably, the thermal transfer element includes a cooling element, which is thermally coupled to the laser assembly and which extracts heat from the laser assembly so as to reduce an overall temperature of at least one of the plurality of optical elements.
Preferably, the cooling element is operated by the stabilizer, and the cooling element extracts heat from the laser assembly responsive to the measured output from the detector.
There is further provided, according to a preferred embodiment of the present invention, a method for stabilizing a laser assembly, the assembly including a plurality of elements each having a respective effective optical length, the plurality of elements forming a cavity resonating at a lasing wavelength in a single mode and having an effective cavity length, the method including:
modulating at least one of the effective optical lengths;
monitoring a radiation output of the assembly responsive to the modulation; and
adjusting the effective cavity length responsive to the output and to the modulation, so as to maintain the cavity resonating at the wavelength in the single mode.
Preferably, modulating the at least one of the effective lengths includes modulating a temperature of at least one of the plurality of elements.
Preferably, modulating the temperature includes providing a heating element which heats at least one of the plurality of elements so as to change the effective length of the at least one of the plurality of elements.
Further preferably, modulating the temperature includes providing a cooling element which cools at least one of the plurality of elements so as to change the effective length of the at least one of the plurality of elements.
Preferably, adjusting the effective cavity length includes adjusting a temperature of at least one of the plurality of elements.
Preferably, adjusting the effective cavity length includes adjusting at least one of the effective optical lengths.
Preferably, modulating the at least one of the effective optical lengths includes measuring a phase of a modulation of the effective optical length, monitoring the radiation output includes monitoring a radiation output phase and evaluating a comparison of the phase of the modulation of the effective optical length with the radiation output phase, and adjusting the effective cavity length includes adjusting at least one of the effective optical lengths responsive to the comparison.
Preferably, adjusting the effective cavity length includes adjusting the length responsive to the monitored radiation output substantially without reliance on an external wavelength reference.
Preferably, the method includes varying a resonant wavelength of at least one of the plurality of elements responsive to the single mode of the cavity.
There is further provided, according to a preferred embodiment of the present invention, laser apparatus, including:
a plurality of optical elements coupled together so as to form a laser cavity resonating in a single mode, one of the plurality of elements having a tunable resonant wavelength; and
a thermal transfer element which is adapted to vary a temperature of the one of the plurality of elements so as to tune the resonant wavelength to correspond with the single mode.
Preferably, the one of the plurality of elements includes a fiber grating.
There is further provided, according to a preferred embodiment of the present invention, laser apparatus, including:
a plurality of optical elements coupled together so as to form a laser cavity resonating in a single mode, one of the plurality of elements having a tunable resonant wavelength; and
a thermal transfer element which is adapted to vary a temperature of the one of the plurality of elements so as to tune the resonant wavelength to correspond with the single mode.
Preferably, the one of the plurality of elements includes a fiber grating.
There is further provided, according to a preferred embodiment of the present invention, laser apparatus, including:
a plurality of optical elements coupled together so as to form a laser cavity resonating in a single mode, a first one of the plurality of elements having a resonant wavelength; and
a thermal transfer element which is adapted to vary a temperature of at least a second one of the plurality of elements, so as to tune the single mode to correspond with the resonant wavelength.
Preferably, the first one of the plurality of elements includes a fiber grating.
There is further provided, according to a preferred embodiment of the present invention, a method for generating a laser output, including:
coupling a plurality of optical elements together so as to form a laser cavity resonating in a single mode, one of the plurality of elements having a tunable resonant wavelength; and
varying a temperature of the one of the plurality of elements so as to tune the resonant wavelength to correspond with the single mode.
Preferably, the one of the plurality of elements includes a fiber grating.
There is further provided, according to a preferred embodiment of the present invention, a method for generating a laser output, including:
a plurality of optical elements coupled together so as to form a laser cavity resonating in a single mode, a first one of the plurality of elements having a resonant wavelength; and
a thermal transfer element which is adapted to vary a temperature of at least a second one of the plurality of elements, so as to tune the single mode to correspond with the resonant wavelength.
Preferably, the first one of the plurality of elements includes a fiber grating.
The present invention will be more fully understood from the following detailed description of the preferred embodiments thereof, taken together with the drawings, in which: